The present invention relates to the art of magnetic resonance imaging of periodically moving tissue. It finds particular application in conjunction with imaging the heart during and immediately following the end-diastole R-wave of the cardiac cycle. However, it is to be appreciated that the present invention also finds application in conjunction with imaging different portions of the cardiac cycle, other tissue which moves in synchronization with the heart, other periodically moving organs or tissues, and the like.
Conventionally, heart images are T.sub.1 weighted spin echo images in which blood is represented in dark and myocardium for defining anatomy is represented in light. The intensity of the signal from blood is a function of two primary factors: (1) flow related spin dephasing and (2) T.sub.1 spin relaxation. The flow of the blood causes spin dephasing and signal loss due to motion through a magnetic field gradient. The magnitude of the blood signal is also related to the T.sub.1 relaxation time and the sequence of repeat time. Artifacts from blood regions of slow flow are a major cause of degradation in cardiac magnetic resonance images.
Presaturation techniques are most commonly used for suppressing blood signals in order to achieve dark blood/bright myocardium images. Presaturation techniques typically include the application of a frequency selective saturation RF pulse of about 90.degree. in the presence of a magnetic field gradient. This converts the longitudinal magnetization in the selected region into transverse magnetization. The selective 90.degree. RF pulse is typically followed by the application of a gradient "spoiler" pulse which serves to dephase the spins in the saturated region such that they contribute no signal to subsequent imaging sequences.
More specifically, blood and other flowing material, e.g. cerebro spinal fluid, in regions or slices outside of the desired imaging region or slice(s) are presaturated with the above described selective 90.degree. RF pulse. A time delay is interposed between the saturation pulse and the imaging sequence which is gauged to allow the saturated spins from outside of the imaging region to flow into the imaging region. The optimal length of this delay is governed by the rate of flow, the vessel geometry, the distance of the presaturation region from the imaging region, and the longitudinal or T.sub.1 relaxation time of the flowing material, and the like.
Although presaturation techniques have been found useful in cardiac imaging, they do have drawbacks. First, a substantial time delay is required between saturation and imaging for effectively eliminating the blood signal from the imaging region. Usually, the presaturation pulse is applied when the R-wave of the patient's cardiac cycle is detected. A delay time long enough to accommodate substantial blood flow into the imaging region is interposed between the presaturation pulse and the start of the image sequences. This blood inflow delay causes a dead time after the R-wave during which image data cannot be acquired. This forecloses the collection of end diastole images and other images during portions of the cardiac cycle immediately following the R-wave. The duration of the inflow delay in human patients is such that the first slice of data is acquired during systole.
Another drawback of saturation methods is the required inclusion of the spoiler gradients. The spoiler gradients are necessary to dephase the signal sufficiently that it does not rephase later during the imaging sequence and contribute to the MR signal during data acquisition. The extra spoiler gradient pulses increase the gradient power requirements of the magnetic resonance system.
Presaturation cardiac images also tend to be degraded by regrowth of the longitudinal magnetization during the inflow accommodating delay time. As the blood inflow delay time is increased, T.sub.1 relaxation results in a regrowth of the longitudinal magnetization and a decrease in the effectiveness of the presaturation. For example, the longitudinal magnetization of blood with a T.sub.1 relaxation time of 500 msec returns to 20% of its equilibrium value about 110 msec after application of the saturation pulse. This leaves a relatively narrow window between the blood inflow delay and 110 msec after the saturation pulse for imaging the heart.
In inversion recovery imaging, a 180.degree. pulse is applied to invert the spin system which is followed after a delay or inversion time by the selected imaging sequence. The delay or inversion time is selected such that some longitudinal or T.sub.1 relaxation occurs before the read out section in which the selected imaging sequence is performed, typically a spin echo or field echo sequence. The signal intensity in the resulting image is a function of primarily the longitudinal or T.sub.1 relaxation time and the inversion time (TI). As the inversion time increases from zero toward the longitudinal relaxation time T.sub.1, the resulting signal starts at a negative maximum and approaches 0. When the delay time reaches 69% (ln 0.5) of the T.sub.1 longitudinal relaxation time, the resulting signal passes through 0. With longer delay times, the signal approaches a positive maximum logrithmically. Stated in absolute value of the magnitude terms, the magnitude of the resultant signal starts at the maximum, decays to zero, and then regrows back toward the maximum. The magnitude of the maximum is affected by the repeat time TR between repetitions of the sequence.
Inversion recovery sequences are commonly used for fat suppression to null signals from fat while collecting image data from other body tissues. The delay time between the inversion pulse and data collection is selected to be near the longitudinal relaxation time for the suppressed fat tissue. In this manner, the signal intensity from the fat is low while the signal from the desired tissue is significantly above the minimum amplitude such that it contributes strongly to the image.
Although inversion recovery techniques have been used in magnetic resonance angiography to differentiate blood from other tissue, they have not heretofore found application in cardiac imaging. For blood with a T.sub.1 relaxation time of 500 msec, the longitudinal magnetization is below 20%, between 255-450 msec after the inversion pulse. If an inversion pulse were triggered by the R-wave, the imaging or read out window would be relatively late in the cardiac cycle.